Previous studies have shown that dangerous high blood pressure is more common in pregnancies from donated eggs, but it has been unclear why.

Experts said latest findings indicate the greater risks are because donated eggs, like transplanted organs or tissue, are not genetically identical to the recipient and probably awaken the immune system.

The placenta formed after egg donation may have more foreign elements and trigger abnormal responses from the mother, which could result in complications, such as miscarriages or high blood pressure.

Eggs from strangers likely trigger a heavier immune reaction than eggs from a relative, experts said, adding that the findings suggest women who need donated eggs might be better off with eggs from a relative.

High blood pressure

In the study, conducted by Dr Cha Sunhwa of Sungkyunkwan University School of Medicine in Seoul, South Korea, 61 pregnancies involving egg donation were compared with a matched group of pregnancies achieved through standard infertility treatment.

Pregnancy-induced high blood pressure, which can become life-threatening to mother and baby, occurred in 12.5% of the women who got donated eggs and in 3.7% of the women who used their own eggs.

The problem was twice as likely to occur following egg donation from a sister, but more than five times as likely following egg donation from a stranger, the study found.